1. Field of the Invention
The present invention relates to a telecommunications network, and in particular, to a spare capacity planning tool to restore a transport network when a single failure event causes multiple span failures.
2. Related Art
A spare capacity planning tool is a tool utilized to plan for service restoration in the event that a full or partial span in a transport network is lost. A transport network typically consists of nodes, links, spans and routes. Links, which carry circuits containing individual telephone lines, for example, are connected together at nodes. The set of all parallel links between two nodes is a span. Spans are connected together to form routes.
If spans are cut or broken, telecommunication service between two points may be interrupted. For example, an entire span or several links within a span may be lost as a result of a natural disaster, such as an earthquake, lightning, or a hurricane. A full or partial span also could be lost because of a human act, such as cutting into buried cables while digging with a backhoe. Because of the manner in which the network is connected, a failure at a single point could cause failures between several locations served by the telecommunications service provider.
If a span in the network is lost, service must be restored. That is, telecommunications traffic on the affected span must be re-routed. Historically, transport network restoration was performed manually by technicians physically disconnecting any failed portions of the network and replacing them with spare portions. In other words, failed portions were bypassed and traffic was re-routed around the failed portions utilizing spares. Today, however, automated restoration techniques are well known in the telecommunications industry, and are described, for example, in U.S. Pat. No. 4,956,835 to Grover and in Grover, W., "Distributed Restoration of the Transport Network," Network Management Into the 21st Century, Chapter 11, IEEE Press (1994).
Today, spare capacity planning is typically handled as a fault management function. The primary goal of fault management is the restoration of service in the presence of faults. As such, fault management must anticipate failures and minimize their occurrence. Proactive fault management strategies favor a network that allows for redundancy. Redundancy permits faulty links (within a span) to be promptly isolated and bypassed by spare links.
To restore telecommunication service, spare capacity is allocated throughout the network. Typically, there is more than one set of links between two nodes in a network. Redundancy is implemented by allocating many of these links as spare capacity. However, spare links constitute idle network capacity. Spare capacity planning tools determine the amount of spare capacity needed to ensure network restorability while controlling the costs associated with excess spare links.
Conventional spare capacity planning tools take into consideration the fact that a failure at a single physical point can affect more than one span. That is, designers of conventional spare capacity planning tools realize as a practical matter that conduits typically contain more than one span from numerous source locations to numerous other destination locations. As such, spare capacity planning tools allow for multiple failures in multiple spans caused by a single cut. Most approaches to spare capacity planning involve a route calculation procedure to find at least one available route in a telecommunications network between any two given points. The available route found (or alternate route) must be capable of carrying all of the circuit-carried information (e.g., telephone calls) that must reach a particular destination.
A typical spare capacity planning tool provides a model for a given telecommunications transport network and offers a proposed allocation of spare links. By simulating network failures and restoration activities within the telecommunication network model, the spare capacity planning tool gauges the adequacy of the proposed allocation of spare links.
A limitation of conventional spare capacity planning tools is that they do not contemplate the sequence in which spans fail in a multiple span failure resulting from a single cut. The sequence in which the spans fail can be an important factor in the restorability of the network. If a spare capacity planning tool does not take into consideration the sequence in which spans fail during a single cut, then the tool may not ensure complete restoration of the transport network. What is needed, therefore, is a spare capacity planning tool that ensures complete restoration of all or substantially all of the failed capacity in the telecommunications transport network regardless of the quantity and sequence of spans that fail.